1. Field of the Invention
This invention relates to a method for recording an optical recording medium of phase change type.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording medium is phase change optical recording medium wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change in reflectance by the crystallographic change is detected for reading of the information. The phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit is simple as compared to magnetooptical recording media.
Most optical recording media of phase change type used chalcogenide systems such as Gexe2x80x94Te system and Gexe2x80x94SBxe2x80x94Te system which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was also recently proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds have been investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb2 or IIb-IVb-Vb2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No.4 (1988), pp. 228. Among the chalcopyrite compounds, AgInTe2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai Nos. (JP-A) 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992. In addition to the optical recording media of phase change type wherein chalcopyrite compounds are used, optical recording media of phase change type wherein AgSbTe2 phase is formed with the crystallization of the recording layer is disclosed in JP-A 267192/1992, 232779/1992, and 166268/1994.
When information is recorded on the optical recording medium of phase change type, the entire recording layer is first brought into crystalline state, and then, a laser beam of high power (recording power) is applied so that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of relatively low power (erasing power) is applied so that the recording layer is heated to a temperature higher than the crystallization temperature and lower than the melting temperature. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallization temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of the phase change type, the medium can be overwritten by modulating the intensity of a single light beam.
In general, recording density of optical recording media including optical recording media of phase change type can be increased to a level higher than that of magnetic recording media. Today, further increase in the recording density is demanded for processing an enormous amount of information as in the case of image processing. Recording density per unit area can be increased by reducing the record mark length, by reducing the interval between the adjacent tracks, and by reducing the interval in the track direction between the adjacent record marks.
However, when the record mark length is reduced, sufficient signal output is not attained, and consequently, C/N will be reduced. The recording medium will also have a narrow recording power margin. Stable formation of the record marks at a predetermined length will also become difficult to result in an increased jitter.
In view of the situation as described above, an object of the present invention is to solve the problems associated with the high density recording at a reduced record mark length.
Such objects are attained by the present invention as described in (1) and (4), below.
(1) An optical recording method for an optical recording medium having a recording layer of phase change type wherein amorphous record marks are formed in the recording layer by irradiating the medium with a recording beam which is power-modulated between high power and low power, wherein minimum record marks are formed such that at least a part of the rear end protrudes toward the forward end of the mark.
(2) An optical recording method according to the above (1) wherein the rear end shape of the record mark protruding toward the forward end is formed by crystallizing the region that has been melted by the irradiation of the recording beam.
(3) An optical recording method according to the above (1) or (2) wherein the minimum record marks are formed such that:
MLxe2x89xa60.4xcex/NA
when ML is length of minimum record marks, xcex is wavelength of the recording beam, and NA is numerical aperture of objective lens of recording optical system.
(4) An optical recording method according to any one of the above (1) to (3) wherein the minimum record marks are formed such that:
xe2x80x83MW/ML greater than 1
when MW is width of minimum record marks and ML is length of minimum record marks.